The field of the invention is systems and methods for ultrasound imaging. More particularly, the invention relates to systems and methods for high resolution ultrasound imaging capable of sub-millimeter resolutions.
There is a need for vascular imaging in the brain that is not met with available clinical imaging modalities. Using computed tomography (“CT”), vessels with a diameter below 400 μm are not consistently detected.
With magnetic resonance imaging (“MRI”) at a field strength of 1.5 T, the limit for vessel detection is approximately 300 μm. With increasing field strength, vessels with smaller diameters can be detected, leading to a greater number of vessels detected at higher field strengths such as 3 T or 7 T. At a field strength of 8 T, vessels estimated to be smaller than 100 μm have been imaged in the human brain. Despite the advances in spatial resolution, MRI remains a costly imaging modality with limited availability, and ultra-high field MRI scanners (e.g., those with field strengths greater than 7 T) that can detect smaller vessels are not found in routine clinical practice. Even at these high field strengths, the ability of MRI to image the smaller vessels that play a key role in many diseases and functions of the brain is limited.
Ultrasound is an imaging modality that does not use ionizing radiation, and that has additional advantages in both its relative low cost and portability. The use of ultrasound in the brain, however, has been severely limited by the attenuating and aberrating effects of the skull bone, which increase with increasing ultrasound frequency. Ultrasound imaging through the skull is thus typically performed at lower frequencies (e.g., 2-4 MHz) through thin acoustic windows in the skull. Because the spatial resolution achievable with ultrasound operating in traditional pulse-echo mode is dependent on frequency, imaging vessels in the brain with this approach sacrifices resolution, which has limited the use of ultrasound to the imaging of major vessels.